A Second Planet Transiting LTT 1445A and a Determination of the Masses of Both Worlds

LTT 1445 is a hierarchical triple M-dwarf star system located at a distance of 6.86 parsecs. The primary star LTT 1445A is known to host the transiting planet LTT 1445Ab with an orbital period of 5.4 days, making it the second-closest known transiting exoplanet system, and the closest one for which the host is an M dwarf. Using TESS data, we present the discovery of a second planet in the LTT 1445 system, with an orbital period of 3.1 days. We combine radial velocity measurements obtained from the five spectrographs ESPRESSO, HARPS, HIRES, MAROON-X, and PFS to establish that the new world also orbits LTT 1445A. We determine the mass and radius of LTT 1445Ab to be 2.87±0.25 M_⊕ and 1.304±0.063 R_⊕, consistent with an Earth-like composition of 33% iron and 67% magnesium silicate. For the newly discovered LTT 1445Ac, we measure a mass of 1.54±0.20 M_⊕ and a minimum radius of 1.15 R_⊕, but we cannot determine the radius directly as the signal-to-noise of our light curve permits both grazing and non-grazing configurations.